All-optical digital signal processing has attracted intensive research interest in recent years, particularly in relation to optical packet-switched communications networks. Various all-optical devices based on nonlinear optical effects, including cross-gain modulation, four-wave mixing and cross-phase modulation have been implemented. These include optical logic gates, optical shift registers, optical cross-connects and optical flip-flop memories, as reported by H. J. S Dorren et al, “Optical packet switching and buffering by using all-optical signal processing methods”, Journal of Lightwave Technology, vol. 21, no. 1, January 2003.